Gemini Observatory: Exploring the Universe, Sharing its Wonders

Eclipsing Stellar Embers Offer Glimpse of Binary Demise

June 14, 2011

Figure 1. The dark curve shows the changing radial velocity (motion toward or away from us) of the primary star through its orbit. Data were obtained using GMOS at the Gemini North telescope on Mauna Kea in Hawai‘i.

The chance alignment of a pair of helium white dwarfs – both originally about the mass of the Sun – is showing how binary stars interact and eventually suffer a long slow demise. The pair, known as CSS 41177, are held in a close gravitational embrace and aligned such that they eclipse each other as seen from the Earth. This serendipitous circumstance allows astronomers to explore the history of these stars, and their conclusions offer a laboratory to test models of stellar evolution and the extreme states of matter that white dwarfs exhibit.

The members of the binary pair, only the second of its kind known, have both endured extreme mass loss as a result of their close pairing. A white dwarf is the usual endpoint of stars like the Sun, but to end as a helium white dwarf, it must lose most of its outer hydrogen mass to reveal only the helium core. This can occur when two stars are tightly paired, as they are in the CSS 41177 system. Each of these stars lost material to its companion’s gravitational pull when it expanded during the red giant stage of its evolution.

The stars orbit each other every 2.8 hours, producing a deep primary eclipse that attenuates 40% of the total light and a secondary eclipse at the 10% level. The orbit is also evident in the Doppler shifts that show the stars’ motion towards and away from the observers through the cycle (Figure 1). The combination of the eclipses and spectral observations yields the precise measurements of mass and radius: each of these helium white dwarfs now has a mass of about 0.3 times the mass of the Sun and a radius of only 0.02 times that of the Sun. White dwarfs are much denser than normal stars, and understanding their mass-radius relationship remains a subject of ongoing investigation into the nature of their unusual state of matter.

The extreme circumstance of the orbiting white dwarfs releases gravitational waves that future generations of detectors may be able to measure. With this loss of energy from the system, the stars are gradually moving towards each other and will merge in just over 1 billion years.

Steven Parsons (University of Warwick, UK) led this work, using observations with the Gemini Multi-object Spectrograph (GMOS) on the Gemini North telescope in Hawai‘i and the RISE camera on the Liverpool Telescope in the Canary Islands. The complete results will appear in The Astrophysical Journal Letters. A preprint is available.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Mauna Kea, Hawai'i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in six partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT), the Australian Research Council (ARC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, and the Brazilian Ministério da Ciência, Tecnologia e Inovação. The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.